Cellular system has become the mainstream of mobile communication systems such as mobile phone systems, in which a plurality of areas (cells), which are ranges within which respective base stations can communicate with mobile stations, are combined together to cover a wider area, and as a mobile station moves, the base station communicating therewith is switched from one to another to enable continued communication.
Currently, third-generation mobile communication services based on the CDMA (Code Division Multiple Access) technology have been launched, and also studies have been actively made on next-generation mobile communication methodology enabling faster communications.
In the 3GPP (3rd Generation Partnership Project), on the other hand, LTE (Long Term Evolution) as well as high-speed wireless service called LTE-advanced, which is an advanced version of LTE, are under review. LTE-advanced plans to introduce relay stations (relays) as a means of achieving high throughput and improving the characteristics in dead areas.
Relay station may be configured in such a manner that mobile stations are unable to recognize the presence of the relay station. In the 3GPP, however, relay stations configured to operate equivalently to ordinary wireless base stations are chiefly under consideration. In this case, for the relay station, a base station at a higher level than the relay station looks as if it were behaving like a mere node such as a router.
As conventional techniques for wireless communication systems including relay stations, a technique has been proposed whereby the amount of signaling is reduced to lessen the frequency of transmissions of a mobile station (Japanese Laid-open Patent Publication No. 2009-81513 (paragraph nos. [0035] to [0047], FIGS. 1 and 2)).
In both LTE and LTE-advanced, base stations often communicate with each other for purposes of handover or interference control. Inter-base station communication interface is defined as X2 interface.
FIG. 9 illustrates such an inter-base station interface. A wireless network 5a includes base stations eNB0 to eNB4. In accordance with the X2 interface of LTE, base stations are connected to each other by a cable. In the illustrated case, the base station eNB0 is connected with the base stations eNB1 to eNB4 through wired transmission paths X2-1 to X2-4, respectively.
For simplicity's sake, only the interface between the base station eNB0 and each of the base stations eNB1 to eNB4 is illustrated. In practice, each base station is connected to every other base station, thus providing a mesh connection.
When communicating with other base stations, the base station eNB0 uses the X2 interface for cable communication. For example, the base station eNB0 uses the wired transmission path X2-1 when communicating with the base station eNB1, and uses the wired transmission path X2-2 when communicating with the base station eNB2.
In this manner, inter-base station communication via the X2 interface is normally effected through a cable. However, where a relay station configured to operate equivalently to a base station exists as stated above, the relay station and a higher-level base station are interconnected wirelessly, and therefore, a wireless interval is included as part of the X2 interface.
FIG. 10 illustrates an inter-base station interface involving a relay station. A wireless network 5b includes base stations eNB0 to eNB4, a relay station RN, and a mobile station UE.
The relay station RN operates in a manner equivalent to ordinary base stations. At a level higher than the relay station RN, a higher-level base station exists (in the illustrated example, the base station eNB0; the higher-level base station is also called Donor), and the relay station RN and the higher-level base station eNB0 are interconnected by a radio propagation path X2-5. The mobile station UE is at a level lower than the relay station RN.
When the relay station RN communicates with any of the base stations eNB1 to eNB4, the communication is performed via the higher-level base station eNB0 and thus involves not only wired communication but wireless communication. For example, when the relay station RN communicates with the base station eNB1, the radio propagation path X2-5 and the wired transmission path X2-1 are used.
Meanwhile, the X2 interface is used also for the transfer (Forwarding) of user data at the time of handover, in addition to the transmission of control information. Forwarding of the user data (hereinafter referred to merely as data) means that a base station as the source of handover transfers, to a base station as the target of handover, data which is not yet completely transmitted to the mobile station.
Such transfer control permits the forwarded data to be transmitted from the target base station to the mobile station when the handover is completed, whereby loss of data due to the handover is prevented.
Where the source base station is a relay station, data is transmitted to the relay station via the higher-level base station. When the data is to be forwarded because of handover, however, the data is again sent back to the higher-level base station and then is forwarded to the handover target base station.
Thus, in the case of a handover involving a relay station as stated above, the data is forwarded by carrying out wireless communication from the higher-level base station to the relay station and wireless communication from the relay station to the higher-level base station. This is, however, undesirable in the light of efficient use of the radio propagation path.
FIG. 11 illustrates the manner of how a handover is executed on a wireless network. The wireless network 5c includes a higher-level base station eNB0, a base station eNB1, a gateway device GW, a relay station RN, and a mobile station UE. The higher-level base station eNB0, the base station eNB1 and the gateway device GW are connected to each other by a wired transmission path. The higher-level base station eNB0 and the relay station RN are connected to each other by a radio propagation path, and the mobile station UE exists at a level lower than the relay station RN.
The thick arrow in FIG. 11 indicates a route along which the data output from the gateway device GW is forwarded when the mobile station UE is handed over from the relay station RN to the base station eNB1.
FIG. 12 illustrates a handover sequence, wherein a handover of the mobile station UE from the relay station RN to the base station eNB1 is executed on the wireless network 5c. Thick arrows indicate flows of data, and thin arrows indicate flows of control information.
S101: The gateway device GW transmits data to the mobile station UE. Specifically, the data is transmitted first to the higher-level base station eNB0, where receiving and transmitting processes are executed and the data is transmitted to the relay station RN. Subsequently, the relay station RN performs receiving and transmitting processes and transmits the data to the mobile station UE.
S102: When executing a handover, the mobile station UE measures the reception levels of radio waves received from nearby base stations and transmits information about the measured reception levels to the relay station RN by including the measurement information in a Measurement Report.
S103: On receiving the Measurement Report, the relay station RN recognizes that the mobile station UE is going to execute a handover, and identifies, based on the reception level measurement information, the base station eNB1 with respect to which a satisfactorily high reception level has been measured, as a candidate for the target of handover.
The relay station RN transmits an HO Request (handover request signaling) to the base station eNB1. The HO Request is transmitted via the higher-level base station eNB0 to the base station eNB1.
S104: On receiving the HO Request, the base station eNB1 determines whether or not the handover is possible. If the handover is possible, the base station eNB1 sends an HO OK (handover permission signaling) as a response. The HO OK is transmitted via the higher-level base station eNB0 to the relay station RN.
S105: Based on the contents of the HO OK, the relay station RN determines the base station eNB1 as the target of handover, and notifies the mobile station UE of the result by means of an HO Command (handover instruction).
S106: After sending a handover instruction to the mobile station UE by means of the HO Command, the relay station RN forwards the data which has been received from the gateway device GW and is not yet completely transmitted to the mobile station UE, to the base station eNB1, which is the target of handover. The data is subjected to receiving and transmitting processes at the higher-level base station eNB0 and then transmitted to the base station eNB1.
S107: On confirming based on the contents of the HO Command that the handover to the base station eNB1 is possible, the mobile station UE executes a handover to the base station eNB1 and transmits an HO Complete (handover completion signaling) to the base station eNB1.
S108: When the HO Complete is received, the base station eNB1 transmits a Path SW, which is data destination switching signaling, to the gateway device GW.
S109: On receiving the Path SW, the gateway device GW switches the destination of data from the higher-level base station eNB0 to the base station eNB1, and transmits the data to the base station eNB1. After the destination is switched, the data is transmitted via the base station eNB1 to the mobile station UE.
In the handover sequence described above, data is exchanged wirelessly between the relay station RN and the higher-level base station eNB0 in Step S106. Such exchange of data increases the volume of wireless communication via the radio propagation path between the relay station RN and the higher-level base station eNB0, with the result that the level of interference from or to other stations rises, giving rise to the problem that the communication quality deteriorates. Also, in the case of wireless communication, processes such as the establishment of a radio link cause a substantial delay, and therefore, a problem also arises in that the processing delay increases if wireless communication takes place frequently.
Although in the above explanation, handover is taken as an example, similar problems can also arise in relation to other communication control procedures.